The optical design of the DCT includes two configurations: a Ritchey-Chrétien focus with a 30 arcmin unvignetted field and interchangeable instruments (the first light configuration) and a prime focus with a 2° FoV camera. The schematic diagram below shows the optical path for both foci.
A ray trace showing both the prime focus and the RC optical paths of the DCT.
Originally we considered a tumbling top end for rapid switching from RC to prime focus. In 2005, the DCT project office commissioned Goodrich, Inc. to conduct a second phase optical design study based on a swappable versus tumbling top end mount configuration. Goodrich’s study resulted in a number of key improvements and cost savings to the DCT optical design:
– Elimination of UV band to allow the use of two normal glasses with a much better index match, resulting in immediate quality improvements to the dispersion correction;
– Number of aspherics reduced to one. This is a simple ellipsoid which can be tested with a conjugate test, if necessary;
– Number of elements has not changed but I-line glasses have been replaced with more commonly available and less expensive materials;
– Better glass choice has resulted in improved performance of the Atmospheric Dispersion Corrector (ADC);
– Performance in all five bands (B, V, R, I, (B+V+R)) has been evaluated and deemed acceptable. No refocusing is required between bands.
For further information on the conceptual optical design, download McFarlane & Dunham 2004, "Optical Design of the Discovery Channel Telescope" [1.26 MB PDF].
FIRST LIGHT INSTRUMENTS